1. Field of the Invention
This invention is in the field of neutron radiography.
2. Description of the Prior Art
Metals have long been subjected to x-ray and gamma ray radiographic analysis. The transmission of such rays through a metal object, however, decreases as atomic weight increases; and, moreover, the degree of transmission through low atomic weight elements differs from that through high atomic weight elements by a factor of only about ten. Hence, it is often difficult to obtain good contrast using x-rays or gamma rays.
More recently, attempts have been made to utilize neutrons in radiographic techniques. While the degree of neutron transmission with atomic weight does not follow a smooth curve, certain high atomic weight materials are as much as 10,000 times more transmissive than certain low atomic weight elements such as hydrogen which is particularly opaque to neutron radiation.
The attenuation of neutrons as they pass through a material is a function of several parameters. In general, the attenuation can be described by the formula: EQU .SIGMA. = N (.sigma..sub.s + .sigma..sub.a),
wherein:
.SIGMA. = attenuation coefficient; PA1 N = atom density of the bombarded material; PA1 .sigma..sub.s = tendency of the given nucleus to "scatter" neutrons passing therethrough; and, PA1 .sigma..sub.a = tendency of the given nucleus to absorb neutrons.
Neutrons, of course, can have a wide range of energy levels and it is known that the attenuation coefficient (.SIGMA.) varies with neutron energy level. A convenient way for specifying neutron energy is in terms of neutron temperature, with absolute zero being equivalent to zero energy. Using this convention, there is a general tendency for materials to become more opaque with lower temperature neutrons. Nevertheless, there are some materials which exhibit a sharp decrease in attenuation coefficient below a certain neutron energy level or temperature, which is often referred to as the "Bragg edge." Below the "Bragg edge," these materials display a decrease in the coherent scattering effect which produces a corresponding drop in their attenuation coefficient (.SIGMA.). Typically, the attenuation coefficient (.SIGMA.) starts to drop off at energies below the "Bragg edge" to a minimum at some discrete temperature after which a slight rise takes place as the energy level is lowered towards absolute zero. For purposes of convenience, neutrons having temperatures in the region of the Bragg edge will be described herein as "cold" neutrons whereas those having temperatures above the Bragg edge will be referred to as "thermal" neutrons (generally above about 0.005 ev).
Cold neutron radiography, i.e., radiography using cold neutrons, has been proposed to analyze iron, which is known to have a Bragg edge. U.S. Pat. No. 3,496,358, to Barton, for example, describes such a technique wherein neutrons produced in a nuclear reactor are used to bombard the iron sample. Attenuation of the cold neutrons by the iron sample is recorded on an imaging system.
The Barton apparatus relies upon a berylium filter cooled with liquid nitrogen to filter leakage neutrons formed in the nuclear reactor so that a beam of neutrons with energies below 0.005 electron-volts can be used to bombard the iron sample. There is no attempt by Barton to form a beam of neutrons at a predetermined temperature, but only an attempt to filter out neutrons having energies above those at which the berylium filter is transparent to neutrons.